In EP 0 743 995 are disclosed various production methods for a dry-formed airlaid material based on dry-laid cellulosic fibres with a suitable binding, e.g. gluing with latex or by means of binder fibres. The laying of the fibres for the formation of a continuous web is usually effected by de-fibering and introducing cellulosic material in an airstream that conveys the loose fibres to a former head above a running perforated formerwire, under which is located a suction box for sucking the fibres down against the wire where they are continuously deposited as a coherent gauze with a desired web thickness. Such products are also termed airlaid non-woven. The products are typically utilised as liquid-absorbing sheets, e.g. by incorporating superabsorbing fibres/granulate. In case that binder fibres are used for stabilising the material, the laid airlaid material can be fixed by passing it through a heating zone. Therefore, thin as well as thick webs can be made as the binder fibres will be equally present in outer as well as inner parts of the material web. As mentioned in EP 0 743 995, the admixing percentage of heat adhesive fibres is limited, typically to about 15%, for economic reasons. Referring to EP 0 202 472 in which is described various airlaid products with bi-component binder fibres, the percentage of which being 15% or 25%, it is proposed as an improvement in EP 0 743 995 in order to reduce the percentage of binder fibres to add a surface impregnation with a layer of glue as this glue fixes the lesser fibres which are not held together by the binder fibres, whereby dust raising from the web is prevented as well.
An airlaid multi-layered product, primarily for wet tissues, with a greater percentage of binder fibres is known from International application WO 01/66345. This document discloses an airlaid fibre web with an inner cellulosic layer between two outer layers. For making the outer layer there is used between 50 and 100% binder fibres and between 0 and 60% cellulosic fibres, and for the inner layer is used between 15% and 40% binder fibres and 50% to 90% cellulosic fibres. In preferred embodiments, no other adhesive is applied whereby the web is only held together by binder fibres, e.g. multi-component fibres or bi-component fibres. A structuring treatment by means of rollers is also mentioned as steps in the production.
An example of making an absorbing multi-layered web without binder is mentioned in EP 2 123 440. Herein is disclosed a cellulosic web as a liquid-absorbing central layer between two outer layers that consist of fibres with a fibre diameter between 50 nm and 25 nm. It is only mentioned as a possibility that in the outer layer there may be provided a polymer with low melting point, preferably between 60 and 130°, in order to adhere to the central layer.
As it appears from the above, various techniques have been developed through the years in order to produce stability in absorbing cellulosic materials. Usually, it is so that a high percentage of binder fibres as in WO 01/66345 provide good stability of the product, though causing high costs and limitations in liquid-absorbing capability, therefore often unwanted, why in EP 0 743 995 there is proposed a lesser percentage of binder fibres supplemented with an outer glue layer, and in EP 2 123 440 it is suggested to completely avoid binder fibres and glue.
The fibre web disclosed in EP 2123440 seems at first to be a good solution, particularly because the relatively expensive synthetic binder fibres are saved, and possible superabsorbing fibres can be utilised to a high degree as they can freely expand during absorption of liquid as opposed to a product with a high percentage of binder fibres, where the latter due to their network reduce utilisation of the superabsorbing fibres because of their reduced expansion possibility. However, it has appeared that a product such as in EP 2123440 tend to fall apart if exposed to larger amounts of liquid; in particular it has appeared that it would be desirable with a better stability of the product if used for collecting liquid in trays for e.g. meat, poultry, fish or similar products, where an aesthetic appearance is crucial and where direct contact between the superabsorbing fibres from the central layer and the meat is ideally to be avoided.
In US 2002/0068081 is described a product and a production method as indicated in the introduction. A very airy airlaid fibre web is produced in this system. No compacting of the fibre web is performed after the formerhead as the web is desired to be maintained as airy as possible at the subsequent heating and the following moisturising. After heating the web is supercalendared to a density of at least 500 kg/m3. The calendaring occurs in a cold calendar unit and the web is cooled before calendaring. Smooth calendar rollers are used, imparting the product a smooth surface without embossings in the surface. Since cold calendaring is applied, it is not possible to press the binder fibres into each other during the calendaring. Supercalendaring of the entire surface of the product have several drawbacks. It requires very high pressure and is therefore energy-consuming. Moreover, great demands are put on dimensioning the facility in order to operate with very large pressures. The pressure strains the facility.
By calendaring, hydrogen bonds are formed between cellulosic fibres in the pulp material. Hydrogen bonds are subsequently broken when a product comes into contact with polar fluids (for example aqueous solutions, body fluids, meat juice and the like). When hydrogen bonds are broken, the product will, as opposed to the above-mentioned product according to EP 2 123 440, still have integrity and not fall apart as the binder fibres are bonded to each other and to some of the cellulosic fibres.
Since cold calendaring is used, a bonding of the outer layer cannot be achieved in this product by means of the binder fibres. Therefore there is a need for a separate fastening technique, e.g. gluing, for fastening the outer layers to the central layer.
By this product is intended a certain drape stiffness. Such stiffness is, however, not required for absorbing products which are intended for placing on a support which is the case when used for collecting liquid in trays from e.g. meat, poultry, fish or similar products, or when used in hygienic products.
The ability of the product to expand caused by liquid absorption depends on the distribution of the binder fibres in the product. The mutual attachment of the binder fibres can thus be limiting to the expansion capability of the product and thereby also a limitation to the amount of liquid that may be absorbed.